1. Field of the Invention
The present invention relates to a process for preparing an open-cell foam structure of a vinyl chloride resin and, more particularly, to a process of preparing an open-cell foam structure by adding a heat decomposable chemical blowing agent to a vinyl chloride resin paste and subjecting the paste to an alkaline decomposition to expand the paste.
2. Description of Prior Art
Previous processes used in preparing an open-cell foam structure of the vinyl chloride resin type have been of two types. In a first process, a vinyl chloride resin paste is used as a base while according to the second technique, a calendering technique is used. The paste technique is described in Japanese patent application No. 21,898/1978 which discloses coating a mixed paste on a base film. The paste is obtained by adding a wax containing from about 20-80 percent carbon constituents, which is incompatible with the vinyl chloride resin. The vinyl chloride resin paste contains azodicarbonamide (AC), a commonly used blowing agent. The resin itself is prepared by means of an emulsion polymerization method. The mixture is foamed and gelled by heating the mixture in a foaming oven to provide an open-cell structure.
The second technique is described in Japanese Laid-Open patent application Nos. 137,930/1980; 70,034/1981; and 90,841/1981. This technique involves the preparation of an open-cell structure by means of a calendering technique. In this technique, AC is used, as in the previous technique, as a blowing agent, and the technique forms pores by means of the thermal decomposition of the blowing agent.
Both of the above techniques provide large cells at the surfaces of the resulting foam (as is seen in FIGS. 3 and 5 of the instant application) as well as large cells throughout the cross-section of the foam (as shown in FIGS. 4 and 6 of the instant application). Since the foams obtained by these methods also possess thin surface layers, the resultant open-cell foam structures have weak surface strength and poor density at their surfaces such that the resulting materials cannot be used as surface sheet material for automobile seats, upholstery material for chairs, or the like.
A process for obviating the above disadvantages which are inherent in conventional techniques has been proposed by Applicant in Japanese Patent Publication No. 28,933/1981. According to this technique, an open-cell foam structure is prepared by means of alkali decomposition of a vinyl chloride resin paste which contains a blowing agent in the presence of an aqueous alkali solution. In this technique, AC is used alone as a blowing agent. However, when using this technique, a portion of the AC remains undissolved as a residue in the resulting open-cell foam structure which exhibits considerable yellowish coloration and provides an extremely low moisture permeability. An increase in the extent of alkali decomposition can somewhat decrease the yellow discoloration, however, this may cause coarse cells to appear on the surface of the resultant foam, and damage the cell structure throughout the cross-section product.